The specific objectives for the coming year are as follows: (1)\The interesting finding that in vivo treatment with a thymic hormone results in accelerated production of interferon in vitro will be pursued. Fr. 5 and thymosin-alpha-1 will be tested for their ability to induce this effect. The dose and time of administration of each thymic hormone preparation will be optimized. Accelerated production of interferon may be caused by an increased number of cells producing interferon, thus reaching the level of detectable interferon faster, or by individual cells producing interferon faster. To discriminate between these possibilities, we will develop the method described in our original grant for enumerating interferon producing cells. This will allow us to determine if the accelerated interferon production is caused by a qualitative or quantitative change in the cells. (2)\The effect of the various thymic hormones on interferon production in vitro will be continued. Variables will include treatment of cultures of spleen cells and thymocytes with thymic hormones for various times before addition of the interferon inducer. In these experiments, a wide range of thymic hormones will be used and interferon will be assayed at several time intervals after addition of the interferon inducer. In addition, all these experiments will also be done in cultures depleted of Ly-1 and Lyt-2 cells, since direct effects on one of these subpopulations of lymphocytes may be masked by competing effects on the other cell type. (3)\One explanation for the variable effects of thymosins on in vivo interferon production is that normal healthy animals have adequate levels of these hormones and, therefore, exogenous addition of thymosin has little or no effect. To test this hypothesis, the in vivo effects of thymosins will be tested in animals pretreated with hydrocortisone. (HF)